For the film thickness quality of shop primer, in addition to meeting the specified average film thickness of 10-15µm, the film thickness should also be uniform, without any missed spots, to ensure the shop primer truly provides rust prevention between processes. When testing the film thickness of shop primer, a magnetic film thickness gauge cannot be used to directly measure the dry film thickness because the surface roughness of the steel plate after shot blasting is 35-50 micrometers, while the shop primer is only 10-15µm; the measured value is not the true thickness of the shop primer.
In actual production, the "small-plate test method" should be used. Several un-shot-blasted and un-painted steel plates are taken, and the thickness of the bare steel plates is measured using a magnetic film thickness gauge. Then, these plates are placed on the pretreatment production line, coated with shop primer, and dried. The total thickness measured minus the thickness of the bare steel plates gives the approximate thickness of the shop primer.
In production, the film thickness of the shop primer is generally controlled by adjusting the dilution ratio, nozzle orifice size, and the steel plate conveying speed. It is particularly important to note that if the workshop primer exceeds 20µm, it will generate a large amount of zinc oxide fumes during cutting and welding, which is detrimental to the health of workers.
